During an inner boring process using a lathe, a boring process using a machining center, or a process using an endmill or a milling cutter is performed, a narrow part or a deep hole part is processed using a long tool, the protruding size of which is relatively large as compared to the total size of the tool. At this time a boring bite or a milling cutter is attached to the tip end of the tool. For this reason, the stiffness at a holder of the cutting tool is lowered as the length of the holder is increased. As a result, chatter vibrations are easily generated, the surface roughness or the accuracy in size is deteriorated, and the tool is damaged or broken.
Various methods have been proposed for preventing the generation of the chatter vibrations when a machining process is performed using such a long tool. For example, there has been proposed a method of improving the stiffness of the tool holder by thermally spraying a material having high Young's modulus, such as cemented carbide, to the outer circumferential part of the tool holder. Also, there has been proposed a method of improving the stiffness of the tool holder by fitting a member, such as cemented carbide, in the tool holder or a groove formed at the inside and the outer circumferential part of the tool holder.
On the other hand, there has also been proposed a method of damping the vibration of the tool by the provision of a damper in which a weight part is supported by elastic members and the vibrations of the weight part are damped using a viscous fluid. For example, Japanese Patent Laid-Open No. H06-31507 discloses a method of using a rod-shaped member as a mechanism for elastically supporting the weight part. Furthermore, Japanese Patent Laid-Open No. S59-110 discloses a method of filling a region surrounded by the outer circumferential surface of a cylindrical weight part and the inner surface of a hollow part of a tool body with a viscous fluid. According to the disclosure of Japanese Patent Laid-Open No. S59-110, ring-shaped elastic members and the weight part are disposed in the hollow part of the tool holder, whereby the vibrations of the tool holder are effectively prevented.
Also, Published Japanese Translation of PCT International Publication for Patent Application No. 06-505322 discloses a method of inserting a coolant pipe through the hollow part of a weight part and filling the gap formed between the coolant pipe and the hollow part with a viscous fluid. According to the disclosure of Published Japanese Translation of PCT International Publication for Patent Application No. 06-505322, the pipe is disposed in a through-hole formed adjacent to the central axis of the weight part, which is formed in the shape of a cylinder, and the weight part is connected to the pipe via ring-shaped elastic members. As a result, the weight is elastically supported with respect to the tool body. Furthermore, the region surrounded by the outer circumferential surface of the pipe and the inner surface of the through-hole is filled with a viscous fluid.
Methods of adding a material having high Young's modulus, such as cemented carbide, to the tool body of a tool provided for preventing the generation of chatter vibrations, which has been proposed from the past, are ones that increase the load necessary to change the spring constant of the tip end of the tool, i.e., the unit length of the tip end of the tool. In these methods, it may be required to form, with high accuracy, grooves or holes, in which the material, such as cemented carbide, is added, and therefore, the manufacturing costs may be increased. Also, it is possible to change the natural frequency by improving the spring constant; however, the damping due to the friction at the connection part is not very large, and therefore, desired vibration prevention is not obtained.
In methods of adding the damper, such as the weight part which is elastically supported, to the tool holder, on the other hand, a dynamic damper is constructed by providing a reaction force, which is proportioned to the vibration amplitude of the tool holder, using the weight part having large inertia, whereby the vibrations are damped. In these methods, the tool body is maintained in a natural frequency mode; however, the natural frequency mode is denied by the dynamic damper, and therefore, the vibration amplitude is decreased. In this method, it is possible to obtain relatively high damping ratio, and therefore, it is considered that this method is the most suitable for improving the stability with respect to the chatter vibrations.
In the boring tool or the rotary tool, it is necessary that the tool body be formed in the shape of a cylinder, and the cylindrical hollow part is formed in the tool body such that a weight part, which is formed substantially in the shape of a cylinder, can be mounted in the cylindrical hollow part. The weight part is connected to the tool body via ring-shaped elastic members, and therefore, the weight part is elastically supported. Consequently, the weight part can be moved relative to the tool body. Also, a viscous fluid is filled between the weight part or the tool body and the supporting mechanism, and therefore, the resistance proportioned to the moving velocity of the weight part is obtained when the weight part is moved relative to the tool body. As a result, the vibrations are damped.
According to the disclosure of Japanese Patent Laid-Open No. 59-110, the viscous resistance when the cylindrical object is moved in the cylindrical viscous fluid is increased as the length and the radius of the fluid layer is increased. Also, the viscous resistance is increased as the gap, i.e., the thickness of the fluid layer is decreased.
In the dynamic damper, the optimum value of the viscous resistance with respect to the movement of the weight part exists, and therefore, it is not possible to obtain the function as the damper when the viscous resistance is too large or too small. The spring constant to support the weight part, is the same. Consequently, in the case that normal silicon oil is used as the viscous fluid, there are frequent occasions when the viscous resistance is too large, or there are frequent occasions when the gap must be large. As a result, in the case that the inner diameter of the damper is fixed, the volume of the weight part mounted in the damper is decreased, and therefore, the performance as the damper is lowered. The optimum value of the viscous resistance of the weight part or the spring constant to support the weight part is greatly changed by the vibration characteristics of the tool body, i.e., the spring constant or the natural frequency of the tool body. In the case that the damper is mounted in the tool, however, the optimum value of the viscous resistance of the weight part or the spring constant to support the weight part is also changed as the length of the tool body or the mass of the cutting tool attached to the tip end of the tool body is changed. In this method, a unit for adjusting the spring constant by compressing the elastic members using screws is disclosed. However, it is necessary to change the outer diameter of the weight part in order to adjust the viscous resistance. It is necessary to change the viscosity of oil filled in the damper or to previously prepare a plurality of weight parts having different outer diameters in order to make sure that the damper responds to various states. For this reason, this method is not suitable for multi-product and small-quantity production applications in which multi-purpose tools are manufactured.
According to the disclosure of Published Japanese Translation of PCT International Publication for Patent Application No. 06-505322, on the other hand, it is necessary to change of the viscosity of the viscous fluid in the damper or to change the inner diameter of the through-hole of the weight part in order to adjust the viscous resistance of the weight part. Since the performance as the damper is improved as the mass of the weight part is increased, it is preferable to make the weight part using a material having high specific gravity. However, a material, such as tungsten, is very expensive. As a result, it is not reasonable to manufacture a plurality of weight parts having different inner diameters in the case that the damper is produced on a large scale. Also, even in the case that the exclusive damper is manufactured only once, it is necessary to prepare a plurality of weight parts having different inner diameters or perform further machining in order to adjust the viscous resistance. For this reason, this method is also not suitable for multi-product and small-quantity production applications in which multi-purpose tools are manufactured.
Also, in the case that the tool body, in which the dynamic damper is mounted, is long, and the mass of the tool body is large, like a long tool, the vibrations of the tool body are easily damped when the mass of the weight part mounted in the tool body is also large. However, when the mass of the weight part is large, the natural frequency in the relative movement of the weight part with respect to the tool body is lowered. Consequently, it is necessary to increase the spring constant of the elastic members in response to the mass of the weight part. It is possible to compress the elastic members in order to change the spring constant. However, the amount of change is not very large, and therefore, it is also possible to decrease the sectional area of the ring-shaped elastic members. However, when the sectional area of the elastic members is decreased, the compression margin is also decreased. As a result, it is difficult to adjust the natural frequency of the weight part.
As described above, according to the conventional arts, in the damper having the structure in which the weight part is supported by the elastic members and the vibrations of the weight part are damped by the viscous fluid, it is necessary to easily change the thickness of the viscous fluid layer in order to optimize the viscous resistance with respect to the movement of the weight part. Also, in the case that the tool body, in which the damper is mounted, is a long tool, i.e., it is necessary that the weight part, the mass of which is large, be mounted in the tool body, the sectional area of the elastic members is decreased, and therefore, the increase of the spring constant is also necessary.